U.S. patent application number 10/648039 was filed with the patent office on 2004-07-22 for variable optical attenuator based on electrically switchable cholesteric liquid crystal reflective polarizers.
Invention is credited to Faris, Sadeg M., He, Zhan.
Application Number | 20040141120 10/648039 |
Document ID | / |
Family ID | 32719802 |
Filed Date | 2004-07-22 |
United States Patent
Application |
20040141120 |
Kind Code |
A1 |
Faris, Sadeg M. ; et
al. |
July 22, 2004 |
Variable optical attenuator based on electrically switchable
cholesteric liquid crystal reflective polarizers
Abstract
Variable optical attenuators (VOAs) are provided, based on
electrically switchable CLC reflective polarizers. The reflectivity
of the described reflective VOAs can theoretically be electrically
adjusted between 0% to 100%, and the reflective bandwidth can be
easily adjusted. In general, the VOA includes a pair of switchable
CLC polarizers and a driver for driving the polarizers. When the
electric fields on both polarizers are off, the polarizers serve as
two reflective mirrors. When the fields are turned on, the two
polarizers are switched into two transparent sheets allowing light
to transmit therethrough. Continuously changing the voltages on the
two polarizers electrically adjusts the attenuation.
Inventors: |
Faris, Sadeg M.;
(Pleasantville, NY) ; He, Zhan; (Chappaqua,
NY) |
Correspondence
Address: |
REVEO, INC.
85 Executive Boulevard
Elmsford
NY
10523
US
|
Family ID: |
32719802 |
Appl. No.: |
10/648039 |
Filed: |
August 26, 2003 |
Related U.S. Patent Documents
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Application
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Filing Date |
Patent Number |
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10648039 |
Aug 26, 2003 |
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09993036 |
Nov 6, 2001 |
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6710823 |
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10648039 |
Aug 26, 2003 |
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09380256 |
Aug 25, 1999 |
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10648039 |
Aug 26, 2003 |
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09032302 |
Feb 27, 1998 |
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6559903 |
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10648039 |
Aug 26, 2003 |
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08805603 |
Feb 26, 1997 |
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5940150 |
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10648039 |
Aug 26, 2003 |
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09354192 |
Jul 15, 1999 |
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6583827 |
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10648039 |
Aug 26, 2003 |
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09093017 |
Jun 5, 1998 |
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6473143 |
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10648039 |
Aug 26, 2003 |
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10281569 |
Oct 28, 2002 |
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10648039 |
Aug 26, 2003 |
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10213523 |
Aug 7, 2002 |
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60406013 |
Aug 26, 2002 |
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Current U.S.
Class: |
349/115 |
Current CPC
Class: |
C02F 2001/46123
20130101; G02F 1/13362 20130101; G02F 1/133365 20130101; G02F
1/13471 20130101; E06B 9/24 20130101; G02F 1/133536 20130101; G02F
1/13718 20130101; F24S 50/80 20180501; E06B 2009/2464 20130101;
Y02E 10/40 20130101; G02F 1/1334 20130101; G02F 1/1393 20130101;
G02F 2203/07 20130101; G02F 1/13345 20210101; G02F 1/133543
20210101 |
Class at
Publication: |
349/115 |
International
Class: |
G02F 001/1335 |
Claims
What is claimed is:
1. A variable optical attenuator comprising: a right-handed
switchable CLC polarizer and a left-handed switchable CLC
polarizers, and an electric driver for driving the polarizers,
wherein when the electric fields on both polarizers are off, the
polarizers serve as two reflective mirrors, which reflect left-hand
circularly polarized light by LH CLC polarizer and right-hand
circularly polarized light by RH CLC polarizer, and wherein when
the fields are turned on, the two polarizers are switched into two
transparent sheets allowing light to transmit therethrough; and
whereby continuously changing the voltages on the two polarizers
electrically adjusts the attenuation.
2. The variable optical attenuator as in claim 1, comprising an
electrical driver for each polarizer.
3. A variable optical attenuator comprising: a first and second
switchable CLC polarizer of the same handedness; a half-wave plate
between the first and second switchable CLC polarizers for
converting the transmitted light from the first polarizer into
opposite handiness, an electric driver for driving the polarizers,
wherein when the electric fields on both polarizers are off, the
polarizers serve as two reflective mirrors, which reflect the same
handedness circularly polarized light by the first polarizer,
converts the opposite handedness circularly polarized light to the
same handedness circularly polarized, and reflects the converted
the same handedness circularly polarized by the second polarizer,
and wherein when the fields are turned on, the two polarizers are
switched into two transparent sheets allowing light to transmit
therethrough; and whereby continuously changing the voltages on the
two polarizers electrically adjusts the attenuation.
4. The variable optical attenuator as in claim 3, comprising an
electrical driver for each polarizer.
Description
RELATED APPLICATIONS
[0001] The present invention is related to U.S. Provisional Patent
Application Serial No. 60/406,013 filed on Aug. 26, 2002 entitled
"Variable Optical Attenuator Based on Electrically Switchable
Cholesteric Liquid Crystal Reflective Polarizers", and also is a
Continuation in Part of U.S. patent application Ser. No. 09/993,036
filed on Nov. 6, 2001 entitled "Electro-Optical Glazing Structures
Having Reflection and Transparent Modes of Operation"; U.S. patent
application Ser. No. 09/380,256 filed on Feb. 25, 1998 entitled
"Electro-Optical Glazing Structures Having Reflection and
Transparent Modes of Operation"; U.S. patent application Ser. No.
09/032,302 filed on Feb. 27, 1998 entitled "Electro-Optical Glazing
Structures Having Reflection and Transparent Modes of Operation";
U.S. Pat. No. 5,940,150 (Ser. No. 08/805,603) filed on Feb. 26,
1997 entitled "Electro-Optical Glazing Structures Having
Total-Reflection and Transparent Modes of Operation for Use in
Dynamical Control of Electromagnetic Radiation"; U.S. patent
application Ser. No. 09/354,192 filed on Jul. 15, 1999 entitled
"Electro-Optical Glazing Structures Having Total-Reflection and
Transparent Modes of Operation for Use in Dynamical Control of
Electromagnetic Radiation"; U.S. patent application Ser. No.
09/093,017 filed on Jun. 5, 1998 entitled "Broadband Switchable
Polarizer", now U.S. Pat. No. 6,473,143; and U.S. patent
application Ser. No. 10/281,569 filed on Oct. 28, 2002 entitled
"Broadband Switchable Polarizer"; all of which are incorporated by
reference herein.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to variable optical
attenuators, and particularly to variable optical attenuator based
on electrically switchable cholesteric liquid crystal reflective
polarizers.
[0004] 2. Description of the Prior Art
[0005] Variable optical attenuators (VOA) play a key role in
current fiber optic communications. The application areas of VOAs
include power level control into receivers, power control into
various optical modules or sub-modules, gain-tilt control or power
equalization in optical amplifier networks.
[0006] Currently there are several forms of VOAs that have been
proposed. An illustration of such proposed VOAs are set forth in
the following: J. M. Hartley, et.al. U.S. Pat. No. 6,253,017 (2001)
disclosing a mechanically rotating VOA; C. E. Lance, et.al. U.S.
Pat. No. 4,516,827 (1985) disclosing a moving optical attenuator
disc; T. Iwakiri, et.al. U.S. Pat. No. 4,893,889 (1990) disclosing
a VOA with an air gap between coupled fiber ends for attenuating
optical power; V. R. Dhuler, et.al. U.S. Pat. No. 6,275,320 (2001)
disclosing a micro-electro-mechanical system (MEMS) VOA; S.
Iwatsuka, et.al. U.S. Pat. No. 5,477,376 (1995) disclosing a
magneto or acoustic optical attenuator; and V. N. Morozov, et.al.
U.S. Pat. No. 6,208,798 (2001) disclosing a VOA with thermo-optic
attenuator and liquid crystal (LC) attenuator. Among the various
forms of VOA, LC based optical attenuators have attracted much
attention due to some unique features such as no moving parts, low
insertion loss and low power consumption.
[0007] Current LC attenuators may be classified into two types:
polarization-control and scattering. An example of
polarization-controlled LC attenuators is as followed: K. Y. Wu,
et.al. U.S. Pat. No. 5,963,291 (1999) disclosing a VOA with a
polarization modulation with a feedback controller; R. Albert,
et.al. U.S. Pat. No. 6,111,633 (2000) disclosing a polarization
independent optical switch for selectively switching an optical
signal; J. J. Pan, U.S. Pat. No. 5,276,747 (1994) disclosing an
optical device that controls the strength of the optical signal; S.
H. Rumbaugh, et.al. U.S. Pat. No. 5,015,057 (1991) disclosing a
polymer-dispersed liquid crystal (PDLC) which provides attenuation
control over attenuation values.
[0008] In polarization-controlled LC attenuators, unpolarized
incident light is usually split by optical crystal or polarizing
beam splitter into two linearly polarized beams with
perpendicularly polarized directions. By transmitting through a LC
cell, the polarization states of the two beams can be controlled by
a voltage applied into the LC cell. Depending on the voltage level,
the amount of light that can be coupled into output fiber can be
adjusted. Thus optical attenuation is electrically achieved.
However, polarization-control based VOAs usually require beam
displacers or polarizing beam splitters to split the incident light
and re-combine them, which causes alignment difficulty and high
cost. Further, the attenuation bandwidths are not easily
adjusted.
[0009] On the other hand, the scattering based LC attenuators
utilize the light scattering effect from a so-called polymer
dispersed liquid crystal (PDLC) device. Such is the occurrence in
W. J. Sinclair, et.al. U.S. Pat. No. 4,364,639 (1982) which
discloses a scattering liquid crystal cell whose optical
transmission can be varied. In such a device, the incident light is
scattered into all directions due to the index-miss-matching
between the LCs and the polymer networks, when no electric field is
applied. Thus, the maximum attenuation ratio can be reached. When a
reasonable high voltage is applied into the device, the LC
molecules are oriented align the electric direction, which causes
disappearance of index-miss-matching. The light can transmit
through the device with minimum insertion loss. However, PDLC
scattering based VOAs cannot totally block the light due to its
scattering effect. The dynamic range usually is small. Further, the
attenuation bandwidths are not easily adjusted.
[0010] Shortcomings of conventional VOAs include: no reflective
mode VOAs; no variability of attenuation bandwidth; high cost; and
difficulty of fabrication. Therefore, a need remains in the art for
reflective mode VOAs, variability of attenuation bandwidth; and
VOAs that are conveniently fabricated.
SUMMARY OF THE INVENTION
[0011] The above-discussed and other problems and deficiencies of
the prior art are overcome or alleviated by the several methods and
apparatus of the present invention for VOAs based on electrically
switchable CLC reflective polarizers. These VOAs are improvements
over conventional VOAs in that:
[0012] There is no need for polarization splitters and
combiners;
[0013] Performance enhancement, as the reflectivity of herein
described reflective VOAs can theoretically be electrically
adjusted between 0% to 100%;
[0014] The reflective bandwidth can be easily adjusted;
[0015] Easy integration;
[0016] Minimal size and weight; and
[0017] Low cost.
[0018] The herein VOA includes: a right-handed switchable CLC
polarizer and a left-handed switchable CLC polarizers, and an
electric driver for driving the polarizers. When the electric
fields on both polarizers are off, the polarizers serve as two
reflective mirrors, which reflect left-hand circularly polarized
light by LH CLC polarizer and right-hand circularly polarized light
by RH CLC polarizer. When the fields are turned on, the two
polarizers are switched into two transparent sheets allowing light
to transmit therethrough. Continuously changing the voltages on the
two polarizers electrically adjusts the attenuation.
[0019] In another embodiment, the herein VOA includes a first and
second switchable CLC polarizer of the same handedness; a half-wave
plate between the first and second switchable CLC polarizers for
converting the transmitted light from the first polarizer into
opposite handiness; and electric drivers for driving the
polarizers. When the electric fields on both polarizers are off,
the polarizers serve as two reflective mirrors, which reflect the
same handedness circularly polarized light by the first polarizer,
converts the opposite handedness circularly polarized light to the
same handedness circularly polarized, and reflects the converted
the same handedness circularly polarized by the second polarizer.
When the fields are turned on, the two polarizers are switched into
two transparent sheets allowing light to transmit therethrough.
Continuously changing the voltages on the two polarizers
electrically adjusts the attenuation.
[0020] The above-discussed and other features and advantages of the
present invention will be appreciated and understood by those
skilled in the art from the following detailed description and
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 shows a CLC film in the helical planar configuration
including a stack of aligned molecular planes, whereby successive
planes are rotated about the z-axis either clockwise or
counterclockwise by an equal amount, tracing either a left-handed
or right-handed helix along the z-axis, wherein the pitch, P.sub.0,
is the thickness of one cyclical stack;
[0022] FIGS. 2A and 2B show optical properties of a right-handed
CLC layer;
[0023] FIG. 3 shows a computer simulation result of an RH CLC
film;
[0024] FIG. 4 illustrates the basic structure of a single
electrically switchable RH CLC reflective polarizer;
[0025] FIG. 5 shows reflectivity of an electrically switchable CLC
reflective polarizer as a function of the applied voltage;
[0026] FIG. 6 illustrates a basic structure of a switchable CLC
polarizer based VOA according to the invention herein;
[0027] FIG. 7 illustrates an alternative embodiment of a switchable
CLC polarizer based VOA according to the invention herein;
DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS
[0028] Herein disclosed is a novel technology to fabricate
electrically variable optical attenuator by using electrically
switchable cholesteric liquid crystal (CLC) polarizers. This type
of CLC polarizers has special cholesteric liquid crystal
polymerized structure with proper low-molecule liquid crystals.
When the electric field is off, the reflectivity for a single layer
in a wavelength range can reach about 50% for unpolarized light. If
two CLC layers with opposite helical handiness are integrated
together, the reflectivity in the narrow reflective band can be
close to 100% theoretically. When an electric field is applied into
the layers, the reflectivity in the reflective bands can gradually
decrease to zero, resulting in full transparent sheets. These CLC
polarizers are described in more detail in U.S. patent application
Ser. No. 09/993,036 filed on Nov. 6, 2001 entitled "Electro-Optical
Glazing Structures Having Reflection and Transparent Modes of
Operation"; U.S. patent application Ser. No. 09/380,256 filed on
Feb. 25, 1998 entitled "Electro-Optical Glazing Structures Having
Reflection and Transparent Modes of Operation"; U.S. patent
application Ser. No. 09/032,302 filed on Feb. 27, 1998 entitled
"Electro-Optical Glazing Structures Having Reflection and
Transparent Modes of Operation"; U.S. Pat. No. 5,940,150 (Ser. No.
08/805,603) filed on Feb. 26, 1997 entitled "Electro-Optical
Glazing Structures Having Total-Reflection and Transparent Modes of
Operation for Use in Dynamical Control of Electromagnetic
Radiation"; U.S. patent application Ser. No. 09/354,192 filed on
Jul. 15, 1999 entitled "Electro-Optical Glazing Structures Having
Total-Reflection and Transparent Modes of Operation for Use in
Dynamical Control of Electromagnetic Radiation"; U.S. patent
application Ser. No. 09/093,017 filed on Jun. 5, 1998 entitled
"Broadband Switchable Polarizer", now U.S. Pat. No. 6,473,143; and
U.S. patent application Ser. No. 10/281,569 filed on Oct. 28, 2002
entitled "Broadband Switchable Polarizer"; all of which are
incorporated by reference herein in their entireties.
[0029] As mentioned above, the reflective VOAs described herein are
based on unique optical properties of CLC films. We will briefly
describe the fundamental of CLC films in general first, and then we
will discuss the electrical switchability of some special CLC
films.
[0030] A CLC film consists of a stack 10 of thousands of molecular
planes, as shown in FIG. 1. Each plane is made of cigar-shaped
liquid crystal molecules 12 that align themselves in a common
direction, denoted by c'. Thousands of these aligned molecular
planes, in turn, stack together so that the orientation of each
molecular plane is rotated slightly from the adjacent plane,
forming a continuous helix. The pitch P.sub.0 of the helix is the
stack thickness needed for the planes to rotate by 360.degree..
[0031] The materials can be formulated to form either a left-handed
(counter-clockwise) helix or a right-handed (clockwise) helix,
which orients perpendicular to the surface of the film. This
helical planar configuration gives rise to unusual optical
properties: the circularly polarized light with the handiness same
as the CLC layer and also the wavelength in the reflective band
determined by the pitch distributions will be totally reflected,
while the light with the opposite handiness, or other wavelengths
will transmit through the layer without any effects.
[0032] The width of the reflection band and the characteristic
wavelength of the selective reflection can be engineered by
altering the composition and processing technique. Referring now to
FIGS. 2A and 2B, a schematic of operation of a film 20 used in
embodiments of the present invention is shown. A thin film 20 with
a right-handed (RH) helical pitch P.sub.0 and average refractive
index n.sub.ave reflects right-circularly polarized light in the
reflection band having a characteristic wavelength:
.lambda..sub.0=n.sub.aveP.sub.0. For example, when a beam 22 of
circularly polarized light in the reflective band impinges on the
film 20, left-circularly polarized light is transmitted 24 at this
wavelength, and right-circularly polarized light is reflected 26 at
this wavelength, functioning as a RH circular polarizer. Note that
overall, 50% of light from beam 22 is reflected and 50% is
transmitted. A left-handed helical pitch film functions similarly
but at opposite handedness. An impinging beam 28 consisting of
right-circularly polarized light out of the reflective band will be
transmitted 30. Further, an impinging beam 32 consisting of
left-circularly polarized light in or out of the reflective band
will be transmitted 34.
[0033] The bandwidth is given by
.DELTA..lambda..apprxeq.(.DELTA.n/n.sub.a- ve).lambda..sub.0, where
.DELTA.n=n.sub.e-n.sub.o is the birefringence of the film. The
bandwidth and position are very easily changed to satisfy different
applications.
[0034] FIG. 3 shows a computer simulation of a CLC film, where the
average index of the CLC material is 1.6, the birefringence
.DELTA.n is 0.1, and the selective reflection wavelength is chosen
at 550 nm. The simulation demonstrates clearly that, in principle,
a high reflection peak at 550 nm is achievable.
[0035] By choosing proper small .DELTA.n materials, the reflection
bandwidth can be adjusted. As the .DELTA.n value is increases, the
bandwidth increases.
[0036] Electrically switchable CLC reflective polarizers may be
formed by choosing proper polymerizable CLC materials, low
molecular materials and other materials. Example of such materials
can be found as followed: J. F. Li, et.al. U.S. Pat. No. 6,473,143
disclosing a polymerized polymer network; R. A. M. Hikmet, U.S.
Pat. No. 5,798,057 (1998) disclosing a mixture of polymerizable.
The basic configuration of electrically switchable CLC reflective
polarizer is similar to the passive CLC layer except for two
transparent and conductive substrates instead of non-conductive
substrates.
[0037] In FIG. 4 the basic structure of an electrically switchable
CLC reflective polarizer 40 is illustrated. The electrically
switchable CLC reflective polarizer 40 includes a CLC right-handed
layer 42; conductive layers 44, 46 on opposite sides of the layer
42; and transparent layers 48, 50 opposite each conductive layer
44, 46, respectively. A DC or AC power circuit is used to drive the
layer.
[0038] The reflective properties of such a CLC layer 42 can be
varied and even eliminated by an external electric field, as shown
in FIG. 5. When V=0, 50% reflectivity for a single CLC layer with
either LH or RH. As increasing the applied voltage V, the
reflectivity decreases from 50% to almost zero. Moreover, such a
switchable behavior is reversible.
[0039] The basic structure of the herein described VOAs is
illustrated in FIG. 6, wherein a VOA 60 is provided, associated
with an input collimator 70 and an output collimator 72 Two
switchable CLC polarizers are provided. A left-handed (LH) CLC
polarizer 62 and a right-handed (RH) CLC polarizer 64. Two electric
drivers 66, 68 are used to drive the polarizers individually. When
the electric fields on both polarizers are off, the two polarizers
perform as two reflective mirrors, which totally reflect left-hand
circularly polarized light by LH CLC polarizer and right-hand
circularly polarized light by RH CLC polarizer. The transmission is
zero theoretically, which could provide a large variable insertion
loss range. When the fields are turned on, the two polarizers are
switched into two transparent sheets. The light can transmit
through them without any losses. The insertion loss (IL) could be
very small. By continuously changing the voltages on the two
polarizers, the attenuation of the device can be electrically
adjusted.
[0040] In an alternative embodiment, and referring now to FIG. 7, a
VOA 70 is provided, associated with an input collimator 80 and an
output collimator 82. Two switchable CLC polarizers of the same
handedness are provided. A pair of right-handed (RH) CLC polarizers
74 are associated with two electric drivers 76, 78 to drive the
polarizers individually. Between the two polarizers, a half-wave
plate 75 is inserted, which converts the transmitted light after
the front polarizer into opposite handiness. The second polarizer
will reflect the polarization-converted light.
[0041] The individual driving gives is advantageous in that the PDL
level may be minimized, since one can adjust the voltage VR/VL or
VR/VR to keep the reflectivity for both handiness lights at the
same levels. Thus, PDL could keep very small in the whole
attenuation range.
[0042] The fabrication process of electrically switchable CLC
reflective polarizer is similar to that for passive CLC reflective
polarizers. The carefully selected CLC materials and non-reactive,
low molecular LC materials with some other materials such as chiral
materials are well mixed. Two transparent and conductive substrates
such as ITO substrates are spin-coated by polymides. After a baking
process, the substrates are rubbed undirectionally. The two
substrates are overlapped together with proper spacers to control
the thickness of the cell. Then, the mixture of LC materials is
filled into the cell. After UV curing treatment, the electrically
switchable CLC reflective polarizer is prepared.
[0043] The selectively reflective wavelength depends on the CLC
materials and also the relative ratios between those materials.
Also, the UV curing condition may change this wavelength. The
bandwidth of the reflective peak is a function of the effective
birefringence and also curing process.
[0044] The mechanism of the switchable behavior can be explained as
follows. When an electric field is applied onto the layer, the low
molecular LCs are oriented by following the electric field, which
will deform the helix structure. As the voltage is increased, the
deformation of the helix structure becomes large, resulting in
decrease of the reflectivity of the layer. When the voltage is high
enough, the helix structure disappears, so that the reflectivity of
the layer drops to zero. Because the helical polymer network has a
little elastic flexibility, the helix structure will appear after
the voltage is removed. Thus, the switching is reversible.
[0045] One of recipes for such an electrically switchable CLC layer
has been published in R. A. M. Hikmet, et.al., Liquid Crystals,
Vol. 26, No. 11 pp. 1645-1653 (1999): 30 wt % chiral acrylate
monomer CBC6, 44 wt % BL64 low molecular LC, 26 wt % chiral
material CB15, and 0.6 wt % dia-acrylate monomer C6M. The
reflective wavelength is about 540 nm and bandwidth is about 50 nm.
The layer can be switched from 32 V and ended at 34 V.
[0046] Using the herein described VAO, the following benefits may
be attained. A reflective mode VOA according to the present
invention has reflectivity properties that can be electrically
changed between 0 and 100%. A controllable band reflective VOAs
according to the present invention can be controlled to cover C
bands and L bands, as used in the optical communication fields. One
embodiment of the reflective VOA utilizes a single handedness.
Another embodiment of the reflective film utilizes two opposite
handedness.
[0047] There are several unprecedented advantages of this
technology over other technologies, including the following:
[0048] Electrically controllable attenuation. Theoretically, the
attenuation in the reflective band can be electrically adjusted
between 0% and 100%.
[0049] Easy integration. No beam displacers or polarizing beam
splitters are needed.
[0050] Minimal size and weight to increase comfort during use.
[0051] Low cost.
[0052] While preferred embodiments have been shown and described,
various modifications and substitutions may be made thereto without
departing from the spirit and scope of the invention. Accordingly,
it is to be understood that the present invention has been
described by way of illustrations and not limitation.
* * * * *